﻿<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body style="background: black;">

    <canvas id="canvas" style="display:block;margin:0 auto;border:1px solid #aaa;">
        您的浏览器尚不支持canvas
    </canvas>

    <canvas id="offCanvas" style="display: none">
    </canvas>

    <script>
        var canvas = document.getElementById("canvas")
        var context = canvas.getContext("2d")

        var offCanvas = document.getElementById("offCanvas")
        var offContext = offCanvas.getContext("2d")

        var image = new Image()
        var isMouseDown = false
        var scale

        window.onload = function(){

            canvas.width = 1152
            canvas.height = 768

            image.src = "img-lg.jpg"
            image.onload = function(){

                offCanvas.width = image.width
                offCanvas.height = image.height
                scale = offCanvas.width / canvas.width

                context.drawImage( image , 0 , 0 , canvas.width , canvas.height )
                offContext.drawImage( image , 0 , 0 )
            }
        }

        function windowToCanvas( x , y ){
            var bbox = canvas.getBoundingClientRect()
            return {x:x-bbox.left , y:y-bbox.top}
        }

        canvas.onmousedown = function(e){
            e.preventDefault()
            isMouseDown = true
            point = windowToCanvas( e.clientX , e.clientY )
            console.log( point.x , point.y )
            drawCanvasWithMagnifier( true , point )
        }

        canvas.onmouseup = function(e){
            e.preventDefault()
            isMouseDown = false
            drawCanvasWithMagnifier( false )
        }

        canvas.onmouseout = function(e){
            e.preventDefault()
            isMouseDown = false
            drawCanvasWithMagnifier( false )
        }

        canvas.onmousemove = function(e){
            e.preventDefault()
            if( isMouseDown == true ){
                point = windowToCanvas( e.clientX , e.clientY )
                console.log( point.x , point.y )
                drawCanvasWithMagnifier( true , point )
            }
        }

        function drawCanvasWithMagnifier( isShowMagnifier , point ){

            context.clearRect( 0 , 0 , canvas.width , canvas.height )
            context.drawImage( image , 0 , 0 , canvas.width , canvas.height )
            if( isShowMagnifier == true ){
                drawMagnifier( point )
            }
        }

        function drawMagnifier( point ){

            var mr = 200

            var imageLG_cx = point.x * scale
            var imageLG_cy = point.y * scale

            var sx = imageLG_cx - mr
            var sy = imageLG_cy - mr

            var dx = point.x - mr
            var dy = point.y - mr

            context.save()

            context.lineWidth = 10.0
            context.strokeStyle = "#069"

            context.beginPath()
            context.arc( point.x , point.y , mr , 0 , Math.PI*2 , false )
            context.stroke()
            context.clip()
            context.drawImage( offCanvas , sx , sy , 2*mr , 2*mr , dx , dy , 2*mr , 2*mr )
            context.restore()
        }

    </script>
</body>
</html>